Typically, a target engine speed is set during idling of an internal combustion engine. To equalize the actual engine speed to the target engine speed, an idle speed control (hereinafter, referred to as ISC) is performed. In the ISC, the opening degree of an intake air amount control valve such as a throttle valve and an idle speed control valve is controlled, thereby adjusting the amount of intake air drawn into a combustion chamber.
During idling of an internal combustion engine, when the load of accessories (for example, an air conditioner compressor and a power steering pump) driven by the power of the engine and the electricity consumption of the vehicle increase, such increase in the load and electricity consumption can suddenly reduce the engine speed. Thus, to compensate for the reduction in the engine speed, the intake air amount needs to be quickly increased. The opening degree of an intake air amount control valve is increased through the ISC.
Since an intake air amount control valve is spaced from a combustion chamber by a certain distance, there is a time lag from when the opening degree of the valve is increased to when the amount of intake air drawn into the combustion chamber is actually increased. During such a delay, the engine speed continues decreasing. In this manner, in the ISC performed by controlling the opening degree of the intake air amount control valve, when the engine speed rapidly decreases due to an increase in the load of accessories and the electricity consumption, the control of the idle speed temporarily lags behind desired timing.
On the other hand, to increase the engine power or to improve the emission, a type of an internal combustion engine has been proposed that includes a variable valve actuation adjustment mechanism (for example, see Japanese Laid-Open Patent Publication No. 2001-263015). Such a mechanism changes valve actuation parameters such as the opening period and the lift of an intake valve.
In an internal combustion engine having such a valve actuation parameter adjustment mechanism, the amount of intake air drawn into a combustion chamber is quickly changed by varying valve actuation parameters. Thus, when the engine speed rapidly drops, the valve actuation parameters are varied in addition to adjustment of increase in the opening degree of the intake air amount control valve, so that the intake air amount is quickly increased.